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DEFENCE IN DEPTH
Defence in depth: reactors

WORKED EXAMPLE: ISOLATION OF CONTAINMENT SPRAY PUMPS

 

Summary: 

  • All four containment spray pumps were isolated for two weeks
  • Following a large LOCA, another system could control containment pressure

Scenario

This two unit station has to shut down both its reactors annually in order to perform the required tests on the common emergency core cooling system (ECCS) and the related automatic safety actions.These tests are usually performed when one of the two reactors is in cold shutdown for refuelling. On 9 October, Units 1 and 2 were subjected to these tests. Unit 1 remained in the cold shutdown condition for refuelling and Unit 2 resumed power operation on 14 October. On 1 November, it was discovered during the monthly check of the safeguard valves that the four valves on the discharge side of the containment spray pumps were closed. It was concluded that these valves had not been reopened after the tests on 9 October, in contradiction to the requirements of the related test procedure. Unit 2 had thus operated for 18 days with spray unavailable.

INES procedure (click the highlighted flowchart items to follow the procedure)

Figure 9, page 150 of the Manual

Determine the safety function operability

The operability of the safety function 'confinement' was degraded.
The operability was less than the minimum required by OL&C but, as a diverse system was available;

The operability was more than just adequate.

Return to flowchart

Determine the initiator frequency

The initiator that would challenge the degraded safety function was a large LOCA.

The initiator frequency is unlikely

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Answer

There was no real initiator, box C3 of Table 10 is appropriate

The isolation of containment spray pumps has a rating of Level 1.